There is a demand for optical amplifiers that can output powers of 1 W or greater, can amplify many wavelength channels simultaneously with low cross-talk and low inter-channel interference, and can do so with high reliability and low cost per wavelength channel.
In many applications such as dense wavelength division multiplexing (WDM) transmission systems and satellite communications, optical amplifiers and transmitters, optically pumped by, e.g., laser diodes, should not only be capable of handling relatively high power but also be protected against failure of pump sources.
Conventional optical amplifiers use single-mode optical fibre whose core is doped with one or more rare-earth ions such as Erbium. Such amplifiers provide limited power output that is insufficient for multi-channel WDM transmission systems. In addition, conventional amplifiers are prone to the failure of pump sources, requiring several pump sources to be contained with the amplifier in order to provide pump redundancy—but at high cost.
The power output of conventional optical amplifiers has recently been increased by the introduction of pump modules containing several semiconductor lasers whose outputs are wavelength division multiplexed into a single optical fibre. Although the output power obtainable from such an optical amplifier containing one of these pump modules is sufficient for amplifying many channels simultaneously, the approach is expensive, is currently limited in powers to around 1 W, and offers limited pump redundancy.
Higher-power optical amplifiers and fibre lasers can be constructed using double-clad optical fibres containing a single-mode waveguiding core doped with rare-earth ions (such as Erbium or Erbium/Ytterbium) and a multi-mode inner cladding formed by the silica cladding guiding against an outer cladding with lower refractive index. This outer cladding is typically a polymer outer cladding. However, it is cumbersome to separate the signal path to the single-mode core from the path required to launch pump powers into the inner cladding. Several techniques have been tried including separating the beams with dichroic mirrors, side pumping using a multimode coupler, and etching pits into the double-clad fibre. However, none of these techniques provides a simple, effective way of reliably introducing the pump energy into the optical amplifier or fibre laser—especially if several pump lasers are required in order to provide pump redundancy. This issue is of concern for high-power fibre lasers where there is a requirement to introduce the pump-energy from several to tens of pump diodes into the laser cavity. No effective, reliable and cost-effective way to achieve this exists in the prior art.
An associated problem is that introducing signal conditioning into the optical amplifier can be difficult. For example, it is often desirable to compensate for the spectral gain variation within the optical amplifier, or to introduce a filter to compensate for the dispersion in a telecommunication link. This requires ready access to the signal, which can be difficult for most amplifier configurations. A requirement therefore exists for an amplifier and laser design where it is simple to insert added functionality.
Today's optical telecommunications networks are increasingly based on wavelength division multiplexing—the simultaneous transmission of many wavelength channels through the same fibre. As the networks expand, these wavelength channels can originate from different locations. This places stringent demands on the management of the network, especially on the performance of optical amplifiers dispersed throughout the network. The wavelength channels arriving at an optical amplifier are unlikely to have equal powers (i.e., they are unbalanced), and the power of an individual wavelength channel can be suddenly and unexpectedly increased. This unbalance and the changing of the power levels in individual channels is referred to as granularity. Prior art optical amplifiers experience problems with unbalanced wavelength channels in that the highest power wavelength channel can be amplified more than the other channels, thus increasing the unbalance. In addition, the sudden changing of the power level in one wavelength channel can cause instabilities in the optical amplifier that carry over to other channels. One of the most robust solutions to remove the granularity is to separate all the wavelength channels prior to amplification, amplify the channels, and then recombine the channels for retransmission. The major problem with this approach is that networks can transmit over one hundred wavelength channels through a single optical fibre. The cost of prior-art optical amplifiers makes this solution unattractive.
The cost issue of optical amplifiers is also a problem as the networks expand into metropolitan areas, the expansion being driven by the insatiable demand for bandwidth for internet, data, mobile phones and cable television. Prior art optical amplifiers are too expensive and this is currently limiting the expansion of the networks.
Erbium-doped fibre amplifiers have revolutionized optical telecommunications over the last ten years. They are finding more and more uses, for instance for compensation of switching losses. The increasing need for capacity in telecommunication networks drives not only amplification requirements, e.g., output power and gain flatness for wavelength division multiplexing applications, but also the required number of amplifiers in a system. Erbium doped fibre amplifiers have remained “stand-alone” devices, with individual amplifiers separately packaged. Component count as well as cost then holds back penetration of the optical amplifiers into different application areas that require a large number of amplifiers at a low cost. Instead, the drive has been towards purpose-built optical amplifiers with high specifications (bandwidth and output power) for use in applications that can tolerate a high cost.
It is therefore an aim of the present invention to obviate or reduce the above mentioned problems.